"If you download potentially copyrighted software, videos or music, your Internet service provider has been watching, and they're coming for you. Specifically, they're coming for you on Thursday, July 12. That's the date when the nation's largest ISPs will all voluntarily implement a new anti-piracy plan that will engage network operators in the largest digital spying scheme in history, and see some users' bandwidth completely cut off until they sign an agreement saying they will not download copyrighted materials." One day, years from now, historians are going to debate whether this was the point of no return.

If that's the case they usually have some tricks like "you agree that we can change the policy any time and etc.". But how far it can go isn't clear.

I'm not sure how far this idea can get in the US. Countries like Germany have laws that state what can legally be in a contract and what cannot. Remember: The thing between a user and his ISP is a contract. The content of this contract has to obey higher laws (e. g. federal law). For example, there is no way a contract that removes my human rights in exchange for ISP service can be legal. Such a clause would immediately disappear from the contract. So even if it was stated in the contract and signed by me, it would be fully meaningless, and there would be no way for the ISP to force me to give up my human rights by that contract.

Nothing that is against the law may be considered legal when agreed to in a contract. So even if you sign a contract to "allow" your ISP to decrypt your network traffic and to cancel your connection when they "think" you're "downloading potentially copyrighted software" and it's against the law, they are not allowed to do it.

Of course, that's my very individual interpretation of how a fair and educated legal system should deal with it; I'm not sure if it fits reality.

The US has the same laws, you cannot surrender a right without complete and full disclosure in a way that someone with a common education can understand, otherwise they are hosed. Contracts cannot absolve anyone from a crime, wiretapping is a crime under federal and state laws without a warrant. Yes ISP's monitoring your data is wiretapping.